Stress is psychosomatic strain caused by various external stimuli that are perceived as burdens. Examples of stress-causing factors (stressors) include physicochemical factors (e.g., cold, heat, noise, and chemical substances); biological factors (e.g., starvation, infection, fatigue, and lack of sleep); and social factors (e.g., mental tension, anxiety, fear, and excitation). In today's complicated society, people are exposed to various types of stressors, and difficulty is encountered in avoiding such stressors in daily life.
When a living body is continuously exposed to a stressor; i.e., when the living body is continuously under stressful conditions, the living body may develop a symptom such as an increase in heart rate or blood pressure, insomnia, rough skin, fatigue, arthralgia, headache, stiff shoulders, asthenopia, anorexia, or constipation. When the living body is further continuously exposed to stressors, the stressors may adversely affect organs of the whole body, resulting in, for example, severe psychosomatic disorder, peptic ulcer, cardiac disease, cerebrovascular disorder, hypertension, or hyperlipidemia.
Conceivably, effective means for coping with such stress is to take an anti-anxiety drug, a hypnotic, or a similar drug, to thereby temporarily alleviate psychosomatic reaction to stressors. However, there has not yet been known an anti-stress drug which has no side effects and can be continuously taken daily.
A benzodiazepine drug, which is a typical anti-anxiety drug, is considered to mitigate, for example, anxiety, tension, depression, or myotonia without affecting the level of consciousness, since the drug acts less directly on the cerebral limbic-neocortical system. However, as has been known, high-dose administration or continuous administration of such a drug causes a withdrawal symptom such as convulsion or delirium, or a side effect such as drowsiness, stagger, dizziness, hepatic disorder, or leukopenia.
Recently, anti-stress compositions containing, for example, a food ingredient have been reported. Specifically, there has been reported a composition containing, for example, β-carotene, L-theanine, astaxanthin, sour milk, gluten, a solvent extract of a plant belonging to the family Ebenaceae, royal jelly, or γ-aminobutyric acid (GABA). Of these substances, particularly, GABA has become of interest as an anti-stress substance (Patent Document 1 and Non-Patent Document 1).
Also, there have been reported a composition containing a plurality of nucleic-acid-related compounds and exhibiting an anti-stress effect, an anti-anxiety effect, or a depression-relieving effect; and an oxidative stress inhibitor containing, as an active ingredient, one or more species selected from among nucleic-acid-related substances including uridylic acid, uridine, and uracil (Patent Documents 3 to 5). Furthermore, there has been reported a method for the treatment of a human with neurological disorders (including stress), the method including administration of an effective dose of uridine or a uridine source (Patent Document 6).    Non-Patent Document 1: Proceedings of Annual Meeting of Japan Society for Bioscience, Biotechnology, and Agrochemistry 2006, Lecture No. 2J16p01-03, page 72    Patent Document 1: JP-A-2002-65175    Patent Document 2: JP-A-2003-327528    Patent Document 3: JP-A-1995-215879    Patent Document 4: JP-A-1998-203989    Patent Document 5: JP-A-2005-330213    Patent Document 6: JP-A-2003-517437